There are real differences between 16 bit and 24 bit files, just as there are real differences between uncompressed and mp3. As with the differences between wav and mp3, 16 bit vs 24 bit is often difficult to identify by listening.

Test signals are one thing, but has anyone found any 24 bit music recording that can be successfully identified via ABX testing against a properly resample to 16 bit version of same?

If I am not mistaken, that is the question we are trying to resolve, whether the arte[/i]facts that can be heard (even with optimal dither) when test tones are used, set at very low levels, and listened to at a high gain setting, can also be heard by the human ear at realistic listening levels, with music.

Well, if that's the question, then to my mind it's already been resolved for 24bit -> 16bit conversions. Consider our 24bit music samples x[n] and dither signal d[n]. Now, we define e[n], which is the difference between the x[n] and quantize_to_16bits(x[n] + d[n]). Ok? Now, optimal dither would do two things:

1) Make e[n] and x[n] completely uncorrelated, and2) Reduce the total power of e[n], while still achieving (1), or3) Reduce the audibility of e[n] while still achieving (1).

From what I understand, for the case of quantization from 24bits (or 32 bits) to 16 bits, the formulation of the jitter signal d[n] to achieve (1) is well understood. The formulation of d[n] to achieve (2) and (3) is a little bit less well understood, but still can be very well approximation with common processes. What (1) means is that there is no distortion at all caused by the quantization process - only the addition of noise.

I think the whole 24 bit/16 bit thing boils down to the hypothesis:Consumer music and movie soundtracks have sufficient dynamic range, that when listened to at "normal" levels, the noise introduced by optimal dithering and quantizing to 16 bits is audible.In my opinion, this hypothesis is yet to be resolved conclusively for real world music and movie soundtracks. This has been demonstrated to be true for some classes of test signals.